This invention relates to a microphone circuit used for a mobile telephone or the like and more particularly, to a microphone circuit having a control switch for changing the communication mode of the telephone.
This kind of microphone circuit is, for example, employed for a mobile telephone designed to accommodate a hands-free mode in which a loudspeaker and microphone are used with subscriber's hands free and a press-to-talk mode in which the loudspeaker and microphone are used alternatively. In the hands-free mode, howling or singing may occur due to crosstalk either in a transformer of a telephone switching office or between the loudspeaker and microphone or both. In order to avoid the singing, the microphone circuit includes a control switch which is manually actuated when the singing occurs, to provide a mode switching signal for changing the hands-free mode in to the press-to-talk mode.
In the conventional microphone circuit, the control switch connects a parallel resistor to or disconnects it from the output of a microphone unit. If the control switch is turned on, the DC bias voltage for the microphone unit decreases by the virtue of the resistor. This decrease of bias voltage is detected by a voltage comparator. The detected output is used as a control signal of mode switching.
Upon turning on the control switch, however, the load impedance of the microphone unit of the conventional microphone circuit decreases due to the resistor, so that not only the bias voltage but also the output signal voltage of the microphone unit decreases. In order to solve this problem, it may be possible to amplify the decreased output of the microphone unit using an amplifier connected to the output terminal of the microphone circuit. This solution increases the circuit size. Moreover, since this solution causes the amplification of the bias voltage in addition to the output signal, the predetermined bias voltage must be again modified. This modification further causes the change of the load impedance of the microphone unit and, therefore, the amplification factor must be modified again. Consequently, it is difficult to adjust the bias voltage and the amplification factor properly.
Another solution of the above problem is to provide a detection signal line for indicating the on/off status of the control switch. However, it means that another signal wire lead must be added to the two-wire lead microphone unit, with the result that the advantage of the two-wire lead microphone unit disappears.